connect both systems in parallel, use half the cathode resistor value which is in now and double the cathode bypass cap. Connect the output coupling cap to the anodes and a plate resistor 5k/10W from anodes to B+.

Alternatively you can try just single systems: Disconnect the upper system in the SRPP stack, or better ground all unused elements. Leave the cathode R and C and use a 10k plate load resistor.

I forgot to add: you should then also change the bias voltage of the heater. As a first try you can simply reference the heater to ground. Or change the voltage divider which elevates the heater to values which will bring it ti about +25V.